Prober: claim only candidates of settled releases, without the join

The settler marks a settled release's candidates ready (and unreadable when
it had no readable name). Claims took the candidates by joining releases
from random points of the queue; in its newer half nothing had settled, so
a slice scanned to the end (0.3 s each, 20 slices and 5 tiers a claim) and
the prober stood still: 2.4M waiting, 0 probed a minute. Now an index over
the claimable rows only, the slices drawn within them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/functional/settler.spec.ts

@@ -24,7 +24,15 @@ test.group('Settler', (group) => {
     })
     const minutesAgo = (m: number) => new Date(Date.now() - m * 60_000)
     await db.table('releases').multiInsert([row('old', minutesAgo(31), posted), row('fresh', minutesAgo(5), posted), row('tail', minutesAgo(31), minutesAgo(40))])
+    const ids = Object.fromEntries((await db.from('releases').select('id', 'name')).map((r) => [r.name, r.id]))
+    await db.table('probe_candidates').multiInsert([
+      { release_id: ids.old, kind: 'lone', message_id: '<a@x>', file: 'a' },
+      { release_id: ids.fresh, kind: 'lone', message_id: '<b@x>', file: 'b' },
+    ])
     assert.equal(await Settler.run(), 1)
+    // The settled release's candidates are claimable now, as unreadable (readable defaults to false).
+    const ready = await db.from('probe_candidates').orderBy('message_id').select('ready', 'unreadable')
+    assert.deepEqual(ready.map((r) => [r.ready, r.unreadable]), [[true, true], [false, false]])
     const settled = (await db.from('releases').whereNotNull('settled_at').select('name')).map((r) => r.name)
     assert.deepEqual(settled, ['old'])
   })